Properties generally required of a secondary battery include (1) high energy density, (2) high power density, (3) low self-discharge rate, (4) reasonable cost, (5) high energy efficiency, and (6) long cycle life (a number of charge and discharge repetition).
Various materials for positive and negative electrodes have been studied and examined for the development of an improved secondary battery with such properties. A lithium secondary battery is known as a high energy density battery having various advantages such as wide applicable temperature range, stable discharge voltage, and very low self-discharge rate. LiCoO.sub.2 having high electromotive force has been proposed as a positive electrode active material for such a high energy density battery (see K. Mizushima et al., MAT. Res. Bull., 15, 783 (1980)). Also, part of Co in LiCoO.sub.2 was substituted by a transition metal, for example, Ni for further improvement [see T. Ohzuku et al. Chemistry Express, 5, 733 (1990)].
When LiCoO.sub.2 oxide is used as a positive electrode active material, a lithium secondary battery including same has a small discharge capacity and poor cycle properties, thus resulting in prominent capacity degradation. On the other hand, when another oxide with part of Co in LiCoO.sub.2 substituted by a transition metal is used as a positive electrode active material, a lithium secondary battery including same has larger discharge capacity but is lower in discharge voltage than that with LiCoO.sub.2. The lower discharge voltage is disadvantageous in obtaining a high energy density battery.
From the foregoing, it is apparent that a lithium secondary battery including either an LiCoO.sub.2 oxide or an oxide with part of Co in LiCoO.sub.2 substituted by a transition metal as a positive electrode active material still has low energy density and poor cycle properties and does not fulfill the requirements in the market.
Another oxide having multi-layer structure and represented by the formula A.sub.x B.sub.y C.sub.z D.sub.w O.sub.2 wherein A is at least one alkali metal, B is a transition metal, C is at least one of Al, In, and Sn; D is at least one of (a) an alkali metal other than A, (b) a transition metal other than B, (c) a IIa group element, and (d) a IIIb, IVb, Vb, or VIb group element of the second through sixth periodic number other than Al, In, Sn, C, N, and O; and x, y, z, and w are respectively 0.05.ltoreq.x.ltoreq.1.10, 0.85.ltoreq.y.ltoreq.1.00, 0.001.ltoreq.z.ltoreq.0.10, and 0.001.ltoreq.w.ltoreq.0.10, has been also proposed as a positive electrode active material which contributes to better cycle properties (Japanese Patent Unexamined Publication No. 63-121258).
The above oxide to be used as a positive electrode active material essentially includes at least one of Al, In, and Sn as the C component, thus improving the cycle properties of the lithium secondary battery. This improved lithium secondary battery, however, does not have sufficient energy density to meet the requirements in the market.